1. Field of the Invention
This invention relates to electrochemical electrodes and particularly to electrochemical reference elements for use in solvents other than water.
2. Description of Related Art
A good example of such usage is ion selective electrodes which are based on organic ion exchangers or neutral carriers. The active material can be in liquid form held in an inert porous membrane but more conveniently the active material is incorporated within a suitably plasticised polymer substrate. In the early designs, the half cell was completed by immersing a silver wire in an internal reference solution of the appropriate chloride salt (potassium for a potassium ion selective electrode) forming a silver/silver chloride internal reference electrode. The potential of this half cell is usually measured against a reference electrode, with the silver/silver chloride reference electrode being usually employed. It will be understood that the ionic current/potential across the membrane is thereby converted into an electronic current/potential at the internal reference electrode, which can be measured. The advantages of using a thermodynamically-definable reversible electrode system as the internal reference electrode will be apparent.
There are clear advantages in developing solid state versions of these ion selective electrodes and considerable efforts have been made in developing so-called "coated wire" electrodes. Commonly, the aqueous internal reference solution is incorporated into a hydrophilic polymer layer such as polyvinyl alcohol (PVA). These devices must be very carefully constructed. For example, in order to prevent osmotic effects across the membrane, this must be introduced when the water content of the PVA layer is within a fixed range. Ideally, this requires coating to be carried out at fixed relative humidities.
In some prior proposals, the use of a reversible electrode has been abandoned with, for example, the membrane being coating directly onto platinum. Even though potentials are measured using very high input impedance volt meters, the ionic to electronic currents are still likely to be high enough to cause problems of drift, noise and lack of reproducibility, in the absence of any thermodynamically reversible electrode. Surface oxides and contamination of the metal can also lead to systematic errors.
A further approach that has been adopted in an attempt to solve this problem is the use of oxidation/reduction processes at noble metal electrodes (for example ferrocence/ferrocenium at a platinum electrode). However, whilst this proposal achieves buffering of the electronic part of the couple, this tends to be unstable.